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Nature. 2018 May;557(7703):101-105. doi: 10.1038/s41586-018-0059-5. Epub 2018 Apr 25.

Deep mitochondrial origin outside the sampled alphaproteobacteria.

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Department of Cell and Molecular Biology, Science for Life Laboratory, Uppsala University, Uppsala, Sweden.
Department of Biology, Utrecht University, Utrecht, The Netherlands.
Department of Medical Biochemistry and Microbiology, Science for Life Laboratory, Uppsala University, Uppsala, Sweden.
Max Planck Institute for Marine Microbiology, Bremen, Germany.
Department of Marine Microbiology and Biogeochemistry, NIOZ Royal Netherlands Institute for Sea Research and Utrecht University, Den Burg, The Netherlands.
Department of Cell and Molecular Biology, Science for Life Laboratory, Uppsala University, Uppsala, Sweden.


Mitochondria are ATP-generating organelles, the endosymbiotic origin of which was a key event in the evolution of eukaryotic cells 1 . Despite strong phylogenetic evidence that mitochondria had an alphaproteobacterial ancestry 2 , efforts to pinpoint their closest relatives among sampled alphaproteobacteria have generated conflicting results, complicating detailed inferences about the identity and nature of the mitochondrial ancestor. While most studies support the idea that mitochondria evolved from an ancestor related to Rickettsiales3-9, an order that includes several host-associated pathogenic and endosymbiotic lineages10,11, others have suggested that mitochondria evolved from a free-living group12-14. Here we re-evaluate the phylogenetic placement of mitochondria. We used genome-resolved binning of oceanic metagenome datasets and increased the genomic sampling of Alphaproteobacteria with twelve divergent clades, and one clade representing a sister group to all Alphaproteobacteria. Subsequent phylogenomic analyses that specifically address long branch attraction and compositional bias artefacts suggest that mitochondria did not evolve from Rickettsiales or any other currently recognized alphaproteobacterial lineage. Rather, our analyses indicate that mitochondria evolved from a proteobacterial lineage that branched off before the divergence of all sampled alphaproteobacteria. In light of this new result, previous hypotheses on the nature of the mitochondrial ancestor6,15,16 should be re-evaluated.

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